Master Linux File Permissions: Users, Groups, and chmod Commands Explained

Linux User Classification

Linux defines two main user types: the superuser ( root) who can perform any operation without restriction, and regular users who have limited capabilities. The superuser prompt is #, while a regular user prompt is $.

File Accessor Classification

Access rights depend on three categories of file attributes:

User (u) : the file’s owner.

Group (g) : the group associated with the file.

Other (o) : all remaining users.

File Types and Permission Basics

When listing files with ls -l, the first character indicates the file type (e.g., d for directory, - for regular file, l for symbolic link, b for block device, c for character device, p for pipe, s for socket).

Permissions are shown in three groups (owner, group, other) using the symbols r (read), w (write), and x (execute). A dash ( -) indicates the absence of a permission.

Symbolic Representation

r--

: read‑only -w-: write‑only --x: execute‑only rw-: read and write r-x: read and execute -wx: write and execute rwx: read, write, and execute ---: no permissions

Octal Representation

Each permission set can be expressed as an octal digit (read = 4, write = 2, execute = 1). For example:

r-- 4 100
-w- 2 010
--x 1 001
rw- 6 110
r-x 5 101
-wx 3 011
rwx 7 111
--- 0 000

Modifying Permissions and Ownership

1) Changing Permissions with chmod

Command format: chmod [options] mode file Options : -R for recursive changes.

Mode syntax can be symbolic (e.g., u+w) or octal (e.g., 664).

Symbolic examples:

chmod u+w /home/abc.txt   # add write permission for owner
chmod o-x /home/abc.txt   # remove execute permission for others

Octal examples:

chmod 664 /home/abc.txt   # owner and group: read/write, others: read
chmod 640 /home/abc.txt   # owner: read/write, group: read, others: none

2) Changing Owner with chown

Command format:

chown [options] newowner file

chown user1 f1   # change owner of f1 to user1

3) Changing Group with chgrp

Command format: chgrp [options] newgroup file These commands allow precise control over who can read, modify, or execute files and directories in a Linux environment.